FUELS (GENERAL METHODS) 301 A totally different formula which permits of the calculation of the calorific value of coals with a high degree of approximation is that proposed by Goutal,l namely : p = SaC + <iV, where p is the calorific power of the fuel as such, C and V are the percentages of fixed carbon (coke less ash) and volatile matter (less moisture) and a a coefficient expressing the calorific power (divided by 100) of the volatile matters and varying with the amount of these volatile matters. To deter- mine the value to be ascribed to a the percentage V1 of volatile matters in the fuel supposed free from moisture and ash is calculated by the formula, V1 =------— ; the corresponding value of a is then obtained from the following table : V1 a V1 a V1 a V1 a Less than 5 IOO 14 I2O 23 105 32 97 5 145 15 117 24 104 33 96 6 142 16 "5 25 103 34 95 7 139 17 113 26 IO2 35 94 8 136 18 112 27 IOI 36 91 9 133 19 no 28 IOO 37 88 10 130 20 ICQ 29 99 38 35 ii 127 21 108 30 98 39 82 12 I24 22 107 31 97 40 80 13 122 2. Calorimetric Determination of the Calorific Power.—The calorimetric or direct methods are undoubtedly to be preferred to those just mentioned as they give far more certain results. The numerous forms of apparatus devised for such determinations may be grouped in three classes : (i) calorimeters in which the combustion takes place in a stream of air or oxygen at the ordinary pressure, like those of Favre and Silbermann, Alexejew, Schwackhofer, and F. Fischer; (2) calorimeters in which the combustion occurs with the aid of an oxidising substance mixed with the fuel, as in those of Lewis Thompson, Stohmann, and Parr ; (3) calorimeters in which the combustion is effected with oxygen at constant volume and very high pressures, known as calorimetric bombs '; the first such bomb was due to Berthelot and Vieille and on this were based the more practical and cheaper ones of Mahler, Hempel and Kroeker, which are the most suitable forms of apparatus for exact determinations. Only the types most commonly used will be described here. (a] LEWIS THOMPSON CALORIMETER. This is a very simple apparatus giving only approximate results comparable among themselves ; it is, however, still in common use in England, where contracts are made on the basis of- its indications. It consists (Fig. 34) of a large glass cylinder with 1 Cowiptes Rendus de I'A cad, des Sciences, 1902, CXXXV, pp. 477-479.p is the required calorific value, referred to the pure fuel.